The present invention relates to a radiation image storage panel favorably employable in a radiation image recording and reproducing method utilizing stimulated emission.
When a stimulable phosphor is exposed to a radiation such as X-ray, it absorbs and stores a portion of the radiation energy. The stimulable phosphor then emits stimulated emission according to the level of the stored energy when the phosphor is exposed to an electromagnetic wave such as visible light or infrared rays (i.e., stimulating light).
A radiation image recording and reproducing method utilizing the stimulable phosphor has been widely employed in practice. The method employs a radiation image storage panel comprising the stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the storage panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with a stimulating light such as a laser beam to emit a stimulated emission; and photoelectrically detecting the emitted light to obtain electric signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
In U.S. patent application Ser. No. 09/749,623, a modified radiation image recording and reproducing method is described. In the method, a function for absorbing radiation and a function for storing the radiation energy are separated, while a stimulable phosphor used in the known radiation image recording and reproducing method performs both functions. For that purpose, a radiation image storage panel comprising at least a stimulable phosphor (for storage) and a fluorescent screen comprising a fluorescent phosphor which absorbs radiation and emits ultraviolet or visible light are used in combination. The method comprises the steps of exposing the fluorescent screen to a radiation having passed through an object or having radiated from an object and converting the radiation into ultraviolet or visible light by the fluorescent phosphor; having the ultraviolet or visible light absorbed by the stimulable phosphor; sequentially scanning a stimulating ray on the stimulable phosphor of the storage panel to emit a stimulated emission; and photoelectrically detecting the emitted light to obtain electric signals of the radiation image.
The radiation image storage panel (often referred to as stimulable phosphor sheet) has a basic structure comprising a support and a stimulable phosphor layer provided thereon. The support may be omitted, if the phosphor layer is self-supporting. On the free surface (surface not facing the support) of the phosphor layer, a protective film is generally placed to keep the phosphor layer from chemical deterioration or physical damage.
The phosphor layer is generally formed by coating a dispersion of phosphor particles in a binder solution on the support and drying the coated dispersion on the support, and therefore comprises a binder and phosphor particles dispersed therein. The binder can be removed by firing the coated phosphor layer to increase a relative density of the phosphor in the phosphor layer.
It is known that the phosphor layer can be also prepared by vapor-accumulating method such as vapor deposition or sputtering.
It is desired that radiation image storage panels used in these methods have sensitivity as high as possible and to provide an image of as high quality (e.g., high sharpness, high graininess) as possible.
Japanese Patent Provisional Publication No. 63-19600 describes a radiation image storage panel having a phosphor layer (film) which is prepared by firing a coated phosphor layer composed of phosphor particles and a binder to have a relative density of 70% or more. The phosphor film generally has a thickness in the range of 20 xcexcm to 1 mm, preferably 50 to 500 xcexcm. In the working examples, phosphor films having a thickness of 200 xcexcm and a relative density of 75 to 93% are prepared.
Japanese Patent Provisional Publication No. 62-39800 describes a radiation image storage panel having a stimulable phosphor film which has cracks extending in its depth direction and is formed by vapor-accumulating method, wherein the phosphor film has a void volume of 3 to 30%. The phosphor film preferably has a thickness in the range of 10 to 800 xcexcm, preferably 50 to 500 xcexcm. In the working examples, phosphor films of RbBr:0.004Tl having a thickness of 250 xcexcm are described.
Japanese Patent Provisional Publication No. H2-58000 describes a radiation image storage panel having a stimulable phosphor film which is produced by vapor-accumulating to form prismatic crystals on a support at a certain angle to the normal direction of the support.
The present invention has an object to provide a radiation image storage panel which has a high sensitivity and which gives a reproduced radiation image having a high sharpness such as MTF of approx. 60% or more (1 c/mm) and MTF of approx. 30% or more (2 c/mm). The radiation image storage panel having such characteristics is very appropriate in the use for medical diagnosis.
As a result of the study performed by the present inventor, it has been discovered that the phosphor film (or layer) formed by vapor-accumulating method should have a very narrow thickness range and a very narrow relative density range, if the phosphor film is directed to the use for medical diagnosis which requires a high sensitivity and a high image sharpness.
The present invention resides in a radiation image storage panel comprising a phosphor film which has cracks extending in a depth direction thereof and is formed by vapor-accumulating method, wherein the phosphor film has a thickness in the range of 300 to 700 xcexcm, preferably 400 to 600 xcexcm, and has a relative density in the range of 85 to 97%, preferably 85 to 95%.
In the radiation image storage panel of the invention, the phosphor film preferably comprises a stimulable alkali metal halide phosphor, specifically a cesium halide phosphor.